The primary goal is to investigate the role of the serine protease, plasminogen activator in neurite outgrowth. Sympathetic neurons and PC 12 cells release the plasminogen activator, urokinase (UK), and a large fraction of the UK released by sumpathetic neurons is from distal processes and growth cones. Inhibitors of UK and affinity-purified antisera directed against UK increase neurite outgrowth from sympathetic neurons (Preliminary Studies). A detailed analysis of this finding will be performed with particular emphasis on the changes in the number and lengths of primary and secondary neurites, alterations in growth cone morphology, its possible substrate dependence, and the role of the active site of UK in this process. A 50 kD protein on the surface of undifferentiated PC 12 cells binds UK, whereas, a 40kD protein on the surface of sympathetic neurons and nerve growth factor-treated PC 12 cells binds UK. The time course for the modulation of these proteins following treatment of PC 12 cells with NGF will be determined, and biochemical studies will be performed to determine if the two proteins are related. The 40 kD protein not only binds UK but also binds a component of the extracellular matrix (ECM). The component in the ECM that binds the 40 kD protein will be determined and the effect of UK on this binding will be measured. Antiserum will be generated against the 40 kD protein and used to study the modulation of this protein by NGF, its interaction with the ECM, and its role in neurite outgrowth. Besides the information gained concerning potentially important molecules involved in neurite outgrowth and regeneration, the studies will provide new data on the interactions between proteases, inhibitors and components of the extracellular matrix, as well as information on proteins that are modulated by nerve growth factor.